1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly to a semiconductor device fabrication method that includes a step of forming a gate oxide film.
2. Description of Related Art
Ordinarily, a semiconductor device that employs silicon carbide crystal is characterized by having a larger energy gap, a larger thermal conductivity, a higher insulation breakdown field strength, and a greater saturated electron drift velocity and suchlike than a conventional semiconductor device that employs silicon crystal. As a result, the application of semiconductor devices that employ silicon carbide crystals as power devices has been foreseen (see, for example, Kazuo Arai and Sadafumi Yoshida, “Fundamentals and Applications of SiC Devices”, Ohmsha, Sep. 30, 2003, pp. 29 to 32). Furthermore, in semiconductor devices that employ silicon carbide crystals, similarly to cases in which silicon crystals are employed, PN-type control is easy, and it is possible to manufacture a thermal oxidation silicon dioxide film, to form a gate oxide film (silicon dioxide film) to serve as an electrode by thermal oxidation of polysilicon doped with P (phosphorus), or the like. Thus, the same processes as when silicon crystals are employed may be applied. Therefore, by replacing silicon crystal with silicon carbide crystal, limits on various device capabilities of conventional semiconductor devices may be raised, and high-performance semiconductor devices may be manufactured.
As methods for forming an oxide film on a silicon carbide substrate, technologies of thermally processing the substrate in an oxidizing atmosphere have been proposed, such as dry oxidation, which is thermal oxidation in an atmosphere of oxygen O2, a wet oxidation method, which is thermal oxidation in an atmosphere in which a small quantity of water vapor H2O has been added to O2, and the like (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-201343 or the like).
However, a silicon carbide substrate differs from a silicon substrate in being a compound of silicon Si and carbon C, and thus fundamentally including carbon as a constituent element. Therefore, it can be understood that carbon that is left in oxide films manufactured in accordance with various oxide film formation conditions affects electronic characteristics, and cases in which oxide films are formed on silicon carbide substrates have problems with interface states, fixed charges and the like. With a 4H-silicon carbide substrate, optimum oxidation conditions vary in accordance with surface orientation. For a silicon surface, dry oxidation at around 1300° C., post-annealing with argon, and H2-annealing are effective to form the gate oxide film with less interface traps and fixed charges. N2O oxidation at 1350° C. or the like is also effective. For a carbon surface, wet oxidation at around 1000° C. is effective.
However, when the same conditions are evaluated with a 3C-silicon carbide substrate, the characteristics are closer to a carbon surface of 4H-silicon carbide, with fixed charges being very numerous with dry oxidation but wet oxidation being effective.
Generally, after a gate electrode is formed, for example, of polysilicon on an oxide film, thermal processing is carried out in order to activate the gate electrode. However, carbon at an interface between the silicon carbide substrate and the gate oxide film will be segregated by this thermal processing. Because of this segregated carbon, interface states are increased and/or positive fixed charges are generated, and as a result a flat band voltage is negatively shifted. Even if the oxide film (gate oxide film) has been formed by wet oxidation as described above, although the amount of shift of a dry oxidation flat band voltage is small, the negative shifting also occurs.
Now, an example of evaluation of CV characteristics is shown in FIG. 6. Agate oxide film is formed by thermal oxidation of a silicon carbide substrate in a wet oxidation atmosphere in a diffusion furnace. Thereon, a polysilicon layer doped with phosphorus to a high density of around 5×1020/cm3 is formed by CVD, a semiconductor device (a MOS capacitor) is formed, and the CV characteristic thereof is evaluated. From FIG. 6, the flat band voltage (Vfb) is around −13 V The threshold (Vt) of a lateral MOS device fabricated by practical application of the conditions of formation of a polysilicon gate electrode with this CV characteristic is −0.5 V, as shown in FIG. 7. The formation of gate oxide film and gate electrode of a MOS capacitor shown in FIG. 6 are applied in the same manner to the device illustrated in FIG. 7, and the CV characteristics exemplify evaluation results of individual gate oxide films of FIG. 7.
Even in a state in which a gate voltage is not applied thus, the current flows in the semiconductor device, and a “normally off” device may not be fabricated.
Generally, even though Vfb is a negative value, if the absolute value of Vfb is small, then the threshold may be adjusted to shift to the positive side by injection of phosphorus or the like into the substrate. However, if the amount of shifting would be large, treatment by this method is not possible.